1. Field of the Invention
The present invention relates to an apparatus for producing a hyperpolarized noble gas, a nuclear magnetic resonance spectrometer using a hyperpolarized noble gas, and a magnetic resonance imager using a hyperpolarized noble gas.
This application claims priority from Japanese Patent Application No. 2005-146029 filed on May 18, 2005, the content of which is incorporated herein by reference.
2. Background Art
Gases at atmospheric pressure have a lower atomic density than liquids and solids, and until recently had not been used as the “targets” in nuclear magnetic resonance spectroscopy (NMR) or magnetic resonance imaging (MRI).
However, when the vapor of an alkali metal such as rubidium (Rb), cesium (Cs) or the like and a noble gas composed of monatomic molecules having a nuclear spin with a spin quantum number of 1/2 such as the xenon isotope having a mass number of 129 (129Xe), the helium isotope having a mass number of 3 (3He) or the like are placed together and are irradiated with a circularly polarized laser to excite the electron spin system (a process called “optical pumping”), the spin system of the noble gas can be polarized (this is known to as “hyperpolarization”).
It has been reported that the NMR signal intensity is enhanced several 10,000-fold in this way, enabling NMR signals to be obtained which are more than 100 times stronger than when the same volume of water is used. This report has drawn attention to NMR/MRI techniques that use hyperpolarized noble gases (see, for example, Patent Document 1).
Here, “hyperpolarization” signifies that the distribution in the spin numbers which occupy the nuclear spin energy levels of an atomic nucleus corresponding to the orientation state with respect to a main static magnetic field is extremely polarized compared with the distribution under a state of thermal equilibrium (Boltzmann distribution).
The process of forming a hyperpolarized noble gas is generally called optical pumping, and works as follows. When an electron at the ground state level of rubidium, for example, is excited by light absorption, jumps to an excited state level, and then returns to the ground state level, it transits with high probability to one of the electron levels of the rubidium ground state levels of which degeneracy has been magnetically broken by an externally applied magnetic field, thereby creating a state of high electron spin polarization in the rubidium molecule. When this rubidium having a highly polarized state collides with a noble gas such as xenon or the like, the highly polarized state of the rubidium is transferred to the nuclear spin system of the noble gas such as xenon or the like, resulting in a hyperpolarized noble gas.
Specifically, as shown in FIG. 5, the nuclear spin energy levels of 129Xe are split by optical pumping, giving rise to an unequal distribution in the number of occupied spin (difference in the number of occupied spin). The magnetic field which is externally applied at the time of such optical pumping is a low magnetic field of about 10−2 T (tesla) (100 gauss). This hyperpolarized 129Xe, instead of being used to carry out measurement in this state, is introduced to a nuclear magnetic resonance spectrometer or a magnetic resonance imager at an even higher magnetic field of about 0.3 T. Thus, the resonance frequency between the two energy levels can be increased with the difference in the number of occupied spin being maintained, enabling the NMR detection sensitivity to be enhanced.
In an NMR/MRI process which uses a hyperpolarized noble gas, unlike an ordinary prior-art NMR/MRI process, measurement does not necessarily involve the integration of NMR signals. Therefore, because the NMR signals are measured only once, it is desirable to supply the hyperpolarized noble gas generated by optical pumping in a hyperpolarized noble gas generating cell to the NMR spectrometer or the magnetic resonance imager in a state in which the difference in the number of occupied spin is maintained.
The phenomenon in which the difference in the number of occupied spin of the hyperpolarized noble gas decreases and approaches the Boltzmann distribution is called “spin relaxation.” The spin relaxation is undesirable because when it occurs, the NMR signal intensity decreases. The main cause of the spin relaxation is thought to be the distortion of electron clouds in the hyperpolarized noble gas due to collisions with the inside wall of the cell or the gas pipeline.
We thus investigated materials of a pipe connected on the downstream side of the hyperpolarized noble gas generating cell, using pipes having an inside diameter of 7 mm. As a result, we found that a Pyrex (trademark) glass pipe is better than a stainless steel pipe or a surface-treated stainless steel pipe because it does not readily give rise to spin relaxation, and we reported our findings (See, for example, Non-Patent Document 1).
However, even with the use of a Pyrex (trademark) glass pipe having an inside diameter of 4 mm, the distance for which the hyperpolarized noble gas can be supplied without allowing spin relaxation to arise is at best somewhat under 1 meter. Hence, it was not possible to supply the hyperpolarized noble gas over a long distance.
In particular, in the case in which a superconducting magnet having a large magnetic field, for example, is used in the NMR spectrometer or the magnetic resonance imager, there is a large leakage magnetic field from the superconducting magnet. This leakage magnetic field apparently has an adverse effect on the hyperpolarized noble gas generating cell located upstream, which lowers the amount of hyperpolarized noble gas generated. Accordingly, it is necessary to have the distance between the hyperpolarized noble gas generating cell and the NMR spectrometer or the magnetic resonance imager be at least 1 m, and preferably 2 to 3 m or more.
Moreover, a glass pipe having an inside diameter of about 4 to 7 mm is more fragile to impacts than a stainless steel pipe, and thus more subject to failure. This, combined with the fact that it cannot be bent, makes it inconvenient to handle for a long-distance supply of the hyperpolarized noble gas.
It is therefore an object of the present invention to provide an apparatus for producing a hyperpolarized noble gas which inhibits spin relaxation and avoids a decline in the NMR signal intensity of the hyperpolarized noble gas even when the gas is supplied over a long distance. Another object of the invention is to provide a nuclear magnetic resonance spectrometer using a hyperpolarized noble gas which includes such an apparatus for producing a hyperpolarized noble gas. A further object of the invention is to provide a magnetic resonance imager using a hyperpolarized noble gas which includes such an apparatus for producing a hyperpolarized noble gas.
(Patent Document 1) Japanese Patent Application, First Publication No. 2003-245263
(Non-Patent Document 1) Moyoko Saito, Takashi Hiraga, Hineyuki Hattori and Toshiharu Nakai, “An investigation of the pipeline materials for continuous hyperpolarized 129Xe gas imaging,” Proceedings of the International Society for Magnetic Resonance in Medicine, Vol. 12 (May 2004), p. 1685